With the ongoing surge of urbanization, a majority of the world's population now resides in urban areas exposed to various environmental stressors. Cities, experiencing temperatures up to 10°C higher than nearby rural areas due to energy consumption and urban infrastructure, necessitate urgent measures to address these challenges, especially in the context of climate change and projected urban population growth. In this landscape, the amplification of tree canopy cover emerges as a potent instrument, with the potential to elevate the quality of urban life significantly. Trees, with their multifaceted benefits—including the reduction of energy consumption, reducing thermal stress and local temperatures via shading and transpiration, mitigation of air pollution, and the overall enhancement of well-being—stand as indispensable contributors. However, the resilience of urban trees is constantly tested by a wide range of abiotic and biotic stressors, accentuated by the formidable impacts of climate change, jeopardizing their functionality, productivity, and survival, reducing their cooling potential and other ecosystem services. Therefore, understanding the intricate relationship between urban environments and the ecophysiology of trees is crucial for addressing climate change, promoting urban forest health, and making informed decisions. To tackle that, the Urban Tree Ecophysiology Network (UTEN) has been established as a global collaboration platform involving researchers, stakeholders, and municipalities. UTEN aims to investigate two fundamental questions: how the urban environment affects tree functionality and health, and how trees modify the microclimate of cities at different biomes. Employing a comprehensive campaign-based approach, accompanied by high-resolution IoT sensors, we continuously measure trees' transpiration, growth, and diameter changes, as well as the surrounding temperature and relative humidity. Additionally, seasonal physiological measurements are employed to assess tree health and functionality. These shared and aggregated data empower researchers to address common questions related to tree health and stress in the face of a changing climate. Furthermore, network nodes can leverage the accumulated knowledge to explore site-specific inquiries tailored to their own urban realities. Through cultivating international collaboration and robust data sharing, UTEN is committed to optimizing the ecosystem services rendered by urban trees. This expansive network, currently encompassing 12 cities across multiple continents, is dedicated to deepening our comprehension of the intricate interplay between trees and the urban environment, thereby paving the way for a more resilient and sustainable urban future. In our presentation, we will share the preliminary outcomes gleaned from approximately one year of meticulous measurements, offering initial insights and preliminary conclusions drawn from these initial findings.

Droughts of increasing severity and frequency are a primary cause of forest mortality associated with climate change. Yet, fundamental knowledge gaps regarding the complex physiology of trees limit the development of more effective management strategies to mitigate drought effects on forests. Here, we highlight some of the basic research needed to better understand tree drought physiology and how new technologies and interdisciplinary approaches can be used to address them. Our discussion focuses on how trees change wood development to mitigate water stress, hormonal responses to drought, genetic variation underlying adaptive drought phenotypes, how trees 'remember' prior stress exposure, and how symbiotic soil microbes affect drought response. Next, we identify opportunities for using research findings to enhance or develop new strategies for managing drought effects on forests, ranging from matching genotypes to environments, to enhancing seedling resilience through nursery treatments, to landscape-scale monitoring and predictions. We conclude with a discussion of the need for co-producing research with land managers and extending research to forests in critical ecological regions beyond the temperate zone.

Woody plant species store nonstructural carbohydrates (NSCs) for many functions. While known to buffer against fluctuations in photosynthetic supply, such as at night, NSC stores are also thought to buffer against environmental extremes, such as drought or freezing temperatures by serving as either back-up energy reserves or osmolytes. However, a clear picture of how NSCs are shaped by climate is still lacking. Here, we update and leverage a unique global database of seasonal NSC storage measurements to examine whether maximum total NSC stores and the amount of soluble sugars are associated with clinal patterns in low temperatures or aridity, indicating they may confer a benefit under freezing or drought conditions. We examine patterns using the average climate at each study site and the unique climatic conditions at the time and place in which the sample was taken. Altogether, our results support the idea that NSC stores act as critical osmolytes. Soluble Sugars increase with both colder and drier conditions in aboveground tissues, indicating they can plastically increase a plants' tolerance of cold or arid conditions. However, maximum total NSCs increased, rather than decreased, with average site temperature and had no relationship to average site aridity. This result suggests that the total amount of NSC a plant stores may be more strongly determined by its capacity to assimilate carbon than by environmental stress. Thus, NSCs are unlikely to serve as reservoir of energy. This study is the most comprehensive synthesis to date of global NSC variation in relation to climate and supports the idea that NSC stores likely serve as buffers against environmental stress. By clarifying their role in cold and drought tolerance, we improve our ability to predict plant response to environment.

The phloem is a key player in whole plant functioning-transporting carbon from sites of production to sites of demand-and is likely influenced by drought due to its dependence on water for generating pressure-driven bulk flow transport. Yet, phloem functioning during drought remains largely unknown due to a lack of experimental studies. Here, we use a phloem-bleeding species, Ricinus communis, to investigate phloem loss-of-function in the context of leaf physiological processes, the mechanisms of phloem turgor maintenance during drought, and the role of turgor in phloem loss-of-function. We found that the solute concentration in the phloem sap doubled over the drought, which allowed phloem turgor to be maintained past the point at which leaves have reached permanent stomatal closure. We also found that phloem turgor did not decline before bleeding ceased, which suggests that phloem bleeding ceassation (interpreted as the cessation of transport) occurred when the phloem still had turgor. In sum, our findings highlight the robustness of phloem functioning, with important implications for forecasting whole-plant carbon dynamics and drought-induced tree mortality.

A robust understanding of phloem functioning in tall trees evades us because current methods for collecting phloem sap do not lend themselves to measuring actively photosynthesizing canopy leaves. We show that Raman spectroscopy can be used as a quantitative tool to assess sucrose concentration in leaf samples. Specifically, we found that Raman spectroscopy can predict physiologically relevant sucrose concentrations (adjusted R-2 of 0.9) in frozen leaf extract spiked with sucrose. We then apply this method to estimate sieve element sucrose concentration in rapidly frozen petioles of canopy red oak (Quercus rubra) trees and found that sucrose concentrations are > 1100 mM at midday and midnight. This concentration is predicted to generate a sieve element turgor pressure high enough to generate bulk flow through the phloem, but is potentially too high to allow for sucrose diffusion from photosynthetic cells. Our findings support the Munch hypothesis for phloem transport once the carbon is in the phloem and challenge the passive-loading hypothesis for carbon movement into the phloem for red oak. This study provides the first similar to in-situ (frozen in the functioning state) source sieve element sucrose concentration characterization in any plant, opening a new avenue for investigation of phloem functioning.

The Fourth Xylem International Meeting (XIM4) brought together over 100 plant scientists to discuss a wide range of topics related to embolism resistance and efficiency of long-distance water transport through plants, and their implications for cultivated and natural systems. The diversity of methods, plant species, and physiological processes discussed in the context of water stress at the meeting highlighted a need to integrate spatial and temporal data about plant functioning, specifically in regard to climate-change-related challenges. Structural traits, such as pit characteristics and conduit diameter, have been traditionally associated with xylem safety and efficiency. Furthering our understanding of these relationships, it was shown that the hydration of pit membranes, their chemical composition, and three-dimensional structure play a key role in the resistance to air-seeding and embolism spread (J. Werner, Ulm University, Germany; Zhang et al., 2020). Resolving conduit network characteristics, such as conduit grouping and intra-organ anatomical trait correlations like those between pit membrane porosity and conduit diameter, was also shown to be important in upscaling xylem anatomy to drought-induced embolism resistance (Mrad et al., 2018). Though conduit diameter remains the easiest anatomical trait to measure, the lack of more comprehensive knowledge on the inter and intraspecific scaling of conduit size and pit traits prevents a more robust understanding of the link between conduit diameter and drought induced (F. Lens, Leiden University, the Netherlands) or freeze-induced embolism resistance (A. Lintunen, University of Helsinki, Finland). The development of noninvasive techniques (e.g. optical vulnerability and synchrotron-based X-ray microtomography) has enabled the study of hydraulics in soft organs, creating a breakthrough in plant vascular research, as demonstrated by the many studies presenting the application of such methods at XIM4. By using noninvasive methods, for instance, a high number of investigations highlighted intraspecific variation in xylem resistance in the stems (C. Lemaire, Université Clermont Auvergne, France; L. Lamarque, University of Bordeaux, France) and leaves (A. Cardoso, Purdue University, West Lafayette, IN, USA; Hochberg et al., 2017) of woody species, both in time (seasonally) and in space (on different locations of the plant). Large intraspecific variation in xylem resistance has also been shown to result in heterogeneous mortality across the canopy in a tree species exposed to drought, with considerable impacts on plant photosynthesis even after rehydration (Cardoso, et al., 2020a). On the contrary, little intraspecific plasticity of functional and anatomical xylem traits was reported for different species across wide aridity gradients when samples were taken at a fixed distance from the apex (E. Robert, CREAF, Barcelona, Spain). Such contrasting results indicate the need to consider known axial trends of anatomical traits when selecting the site on a branch/stem axis for physiological experiments to avoid potential bias in the interpretation of results (Lechthaler et al., 2019). The use of more rigorous sampling protocols and the new, noninvasive methods will likely lead to further insights into the phenotypic plasticity of xylem's anatomical and functional traits that plants may develop as a response to future climate changes. A number of XIM4 contributions emphasized that, in order to build a holistic picture of the hydraulic processes controlling plant responses to drought, hydraulics at the extremities of the water transport system (i.e. soil–roots and leaves–atmosphere) must be further investigated. The presence of biochemical barriers at both root–soil and leaf–atmosphere interfaces (suberized cell walls of exo and endodermis in roots and extracellular biopolymers of the leaf cuticle, respectively) likely play a key role in water transport efficiency and safety, as they reduce the free diffusion of water and nutrients (Schreiber, 2010). Moreover, declines in soil hydraulic conductivity and root hydraulic conductance during moderate drought have been shown to be linked to the disconnection between roots and the rhizosphere (Rodriguez-Dominguez & Brodribb, 2020), with consequent limitations to leaf transpiration (A. Carminati, University of Bayreuth, Germany). At the other extremity of the hydraulic path, increasing vapor pressure deficit in the atmosphere is long known to be tightly associated with leaf water losses through stomata (McAdam & Brodribb, 2015) and the leaf cuticle (L-M. Billon, Université Clermont Auvergne, France). A better integration of the hydraulics at the extremities in the modeling of plant–water relations will increase our ability to understand plant behavior in the context of water stress. Hydraulic failure due to xylem embolism is recognized as a major trigger of tree mortality under drought (Choat et al., 2018). Plant species display different mechanisms to avoid air-seeding and subsequent hydraulic failure. Declines in stomatal conductance were shown to preserve leaf and stem xylem from embolism in a herbaceous grass (D. Corso, University of Tasmania, Australia) and in woody angiosperms (Creek et al., 2019), as well as in a pair of lycophyte species (S. McAdam, Purdue University, West Lafayette, IN, USA), suggesting an evolutionary trait coordination across land plants (Cardoso, et al., 2020b). Indeed, a trait-based model showed that the time of plant desiccation was longer with wider differences between the water potential at 50% loss in stem hydraulic conductance and stomatal closure (Blackman et al., 2019). Nevertheless, water loss through the cuticle (gmin) was shown to continue well beyond the point of stomatal closure, potentially leading to further increases in xylem tension during prolonged droughts (Duursma et al., 2018). Owing to the importance of leaf water loss to the whole-plant water status, lowering gmin was proposed as an important mechanism preventing the development of excessively low water potentials during drought (L-M. Billon). Additionally, reversible collapse of minor veins in leaves has also been suggested to effectively buffer embolism in major veins in red oak leaves (N. Holbrook, Harvard University, Cambridge, MA, USA; Zhang et al., 2016). It has long been debated whether plants can refill embolized conduits with water, or if embolized conduits are permanently lost. Through the direct observation of embolism via noninvasive methods, refilling under tension has been shown to be an artefact. However, xylem refilling upon rewatering remains uncertain. Lack of refilling upon rewatering in intact plants was found in leaves of a herbaceous grass (Johnson et al., 2018) and stems of two woody species (L. Lamarque, University of Bordeaux, France; R. Rehschuh, Karlsruhe Institute of Technology, Germany), and it has been suggested to occur in silver birch (Y. Salmon, University of Helsinki, Finland). This suggests that xylem refilling upon rewatering may be possible in some species and plant organs but not in others. The formation of new xylem every year plays a key role in plant responses to environmental cues, as a central process in stress acclimation (E. Ziaco, University of Nevada, Reno, USA; G. Battipaglia, University of Campania, Caserta, Italy) and recovery (J. Gričar, Slovenian Forestry Institute, Ljubljana, Slovenia). Growing areas of research on cambial activity and retrospective dendro-anatomy (i.e. the study of time series of wood anatomical traits) are expanding the time resolution of our analyses of xylem physiology, extending our understanding of how intra and interannual environmental variability affects the xylem hydraulic functioning. Alongside hydraulic failure, carbon (C) starvation is an important factor contributing to plant damage during drought, and a combination of these processes has been proposed to result in a cascade of events that ultimately lead to plant mortality (N.G. McDowell, Pacific Northwest National Laboratory, Richland, WA, USA). The association between hydraulic failure and C starvation may be due to the role of carbohydrate supply for preventing and repairing xylem embolism, as well as the role of carbohydrates in regrowth following drought relief and osmotic regulation (McDowell et al., 2019). The interplay of C and water dynamics is a novel frontier for plant physiologists, and novel techniques such as Raman spectroscopy (enabling the measurement of sucrose at the cellular level; J. Gersony, Harvard University, Cambridge, MA, USA) could help us better understand phenomena like drought-induced tree mortality. The plant hydraulic community is aware of climate change threats – see the manifesto signed by all conference participants in Cochard et al. (2019). Our ability to predict, and therefore mitigate, the negative effects of climate change in plants from both cultivated and natural systems relies on a deep and comprehensive knowledge of the plant hydraulic system as a whole. A better understanding of plant water relations is also critical for predicting the contribution of forest ecosystems to the global C and water cycles by implementing the information of plant hydraulics into climate models. To achieve this, several gaps in our understanding of basic plant functioning still need to be filled. Whereas most experimental plant hydraulic studies have been performed using potted plants under controlled environments, it is clear from XIM4 that our current understanding would benefit from experiments on plants in their natural environment. Such experiments would allow us to access the whole soil–plant–atmosphere continuum and the interaction between the different components of this continuum during drought. Robust studies in the field are also important in improving our ability to scale-up plant processes from the individual to the ecosystem level. Significant recent progress has been made in developing new technologies to assess drought damage, understanding the traits associated with drought resistance and building a more holistic picture of plant resistance to drought. However, a number of important questions surrounding plant hydraulics have been highlighted at XIM4, such as the role of carbohydrate metabolism in drought physiology, the existence and importance of post-drought refilling, the intraspecific variation in plant hydraulic traits, and the interaction of different components of the soil–plant–atmosphere continuum. New tools and methodologies presented at XIM4 provide us with the opportunity to address some of these critical knowledge gaps to improve our understanding of plant biology with implications for industry and broader society in the face of global climate change. We thank the participants of XIM4 for their scientific contributions to the meeting. Participation of KMJ, JTG, AM, CL and AFB in XIM4 was supported by Federation of European Societies of Plant Biology. Participation of L-MB, YW, AG, AAC and LF-d-U was supported by the New Phytologist Trust. AAC wrote the report draft based on the contributions of all authors.

Warming-induced nutrient enrichment in the Arctic may lead to shifts in leaf-level physiological properties and processes with potential consequences for plant community dynamics and ecosystem function. To explore the physiological responses of Arctic tundra vegetation to increasing nutrient availability, we examined how a set of leaf nutrient and physiological characteristics of eight plant species (representing four plant functional groups) respond to a gradient of experimental nitrogen (N) and phosphorus (P) enrichment. Specifically, we examined a set of chlorophyll fluorescence measures related to photosynthetic efficiency, performance and stress, and two leaf nutrient traits (leaf %C and %N), across an experimental nutrient gradient at the Arctic Long Term Ecological Research site, located in the northern foothills of the Brooks Range, Alaska. In addition, we explicitly assessed the direct relationships between chlorophyll fluorescence and leaf %N. We found significant differences in physiological and nutrient traits between species and plant functional groups, and we found that species within one functional group (deciduous shrubs) have significantly greater leaf %N at high levels of nutrient addition. In addition, we found positive, saturating relationships between leaf %N and chlorophyll fluorescence measures across all species. Our results highlight species-specific differences in leaf nutrient traits and physiology in this ecosystem. In particular, the effects of a gradient of nutrient enrichment were most prominent in deciduous plant species, the plant functional group known to be increasing in relative abundance with warming in this ecosystem.

Trees typically experience large diurnal depressions in water potential, which may impede carbon export from leaves during the day because the xylem is the source of water for the phloem. As water potential becomes more negative, higher phloem osmotic concentrations are needed to draw water in from the xylem. Generating this high concentration of sugar in the phloem is particularly an issue for the ∼50% of trees that exhibit passive loading. These ideas motivate the hypothesis that carbon export in woody plants occurs predominantly at night, with sugars that accumulate during the day assisting in mesophyll turgor maintenance or being converted to starch. To test this, diurnal and seasonal patterns of leaf nonstructural carbohydrates, photosynthesis, solute, and water potential were measured, and carbon export was estimated in leaves of five mature (>20 m tall) red oak (Quercus rubra) trees, a species characterized as a passive loader. Export occurred throughout the day at equal or higher rates than at night despite a decrease in water potential to -1.8 MPa at midday. Suc and starch accumulated over the course of the day, with Suc contributing ∼50% of the 0.4 MPa diurnal osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always >0.7 MPa. These findings illustrate the robustness of phloem functioning despite diurnal fluctuations in leaf water potential and the role of nonstructural carbohydrates in leaf turgor maintenance.

AbstractThe influence of temperature on diversity and ecosystem functioning is well studied; the converse however, that is, how biodiversity influences temperature, much less so. We manipulated freshwater algal species diversity in microbial microcosms to uncover how diversity influenced primary production, which is well documented in biodiversity research. We then also explored how visible‐spectrum absorbance and the local thermal environment responded to biodiversity change. Variations in the local thermal environment, that is, in the temperature of the immediate surroundings of a community, are known to matter not only for the rate of ecosystem processes, but also for persistence of species assemblages and the very relationship between biodiversity and ecosystem functioning. In our microcosm experiment, we found a significant positive association between algal species richness and primary production, a negative association between primary production and visible‐spectrum absorbance, and a positive association between visible‐spectrum absorbance and the response of the local thermal environment (i.e., change in thermal infrared emittance over a unit time). These findings support an indirect effect of algal diversity on the local thermal environment pointing to a hitherto unrecognized biodiversity effect in which diversity has a predictable influence on local thermal environments.

Current conceptions of sucrose export largely neglect the effect of transpiration-induced water potential gradients within leaf mesophyll, even as the mix of convection and diffusion in the pre-phloem path remains uncertain. It is also generally held that the relative importance of convection and diffusion in the pre-phloem path is controlled by the ratio of their respective mass transfer coefficients. Here, we consider pre-phloem sucrose transport in the presence of adverse water potential gradients, finding that whether convection impedes or aids sucrose delivery to the phloem is independent of the permeability of the plasmodesmata to bulk flow, and depends only on assimilation rate, path-length, and the diffusivity. For most tissues subject to transpiration, convection through plasmodesmata pushes sugar away from the phloem.

As global climatic changes increase plant susceptibility to large-scale disturbances such as drought and pathogens, understory responses to these disturbances will become increasingly important to long-term forest dynamics. To better understand understory responses to canopy disturbance, we measured changes in the growth and physiology of the dominant understory shrub, American witch-hazel (Hamamelis virginiana L.), in response to girdling of canopy oaks in a temperate hardwood forest of the northeastern United States. Changes in the growth and physiology of H. virginiana may be important to the regeneration of northeastern temperate forests, as this common shrub largely shapes the microenvironment for seedlings on the forest floor where it occurs. Canopy disturbance by girdling resulted in significant increases in light and soil nitrogen availability. In response to these environmental changes, basal-area growth of H. virginiana increased by an average 334%. This growth increase corresponded to significant increases in foliar nitrogen, respiration, and leaf chlorophyll and carotenoid concentrations. These findings indicate improved environmental conditions and increased growth for this understory shrub following the loss of dominant canopy trees. This study suggests that following large-scale canopy disturbance, H. virginiana and shrubs like it may play an important role in competing for soil N and shading seedlings of regenerating canopy species.

Trees present a critical challenge to long-distance transport because as a tree grows in height and the transport pathway increases in length, the hydraulic resistance of the vascular tissue should increase. This has led many to question whether trees can rely on a passive transport mechanism to move carbohydrates from their leaves to their roots. Although species that actively load sugars into their phloem, such as vines and herbs, can increase the driving force for transport as they elongate, it is possible that many trees cannot generate high turgor pressures because they do not use transporters to load sugar into the phloem. Here, we examine how trees can maintain efficient carbohydrate transport as they grow taller by analysing sieve tube anatomy, including sieve plate geometry, using recently developed preparation and imaging techniques, and by measuring the turgor pressures in the leaves of a tall tree in situ. Across nine deciduous species, we find that hydraulic resistance in the phloem scales inversely with plant height because of a shift in sieve element structure along the length of individual trees. This scaling relationship seems robust across multiple species despite large differences in plate anatomy. The importance of this scaling becomes clear when phloem transport is modelled using turgor pressures measured in the leaves of a mature red oak tree. These pressures are of sufficient magnitude to drive phloem transport only in concert with structural changes in the phloem that reduce transport resistance. As a result, the key to the long-standing mystery of how trees maintain phloem transport as they increase in size lies in the structure of the phloem and its ability to change hydraulic properties with plant height.

The time scale of stomatal closure and xylem cavitation during plant dehydration, as well as the fate of embolized organs, are under debate, largely due to methodological limitations in the evaluation of embolism. While some argue that complete stomatal closure precedes the occurrence of embolism, others believe that the two are contemporaneous processes that are accompanied by daily xylem refilling. Here, we utilize an optical light transmission method to continuously monitor xylem cavitation in leaves of dehydrating grapevine (Vitis vinifera) in concert with stomatal conductance and stem and petiole hydraulic measurements. Magnetic resonance imaging was used to continuously monitor xylem cavitation and flow rates in the stem of an intact vine during 10 d of dehydration. The results showed that complete stomatal closure preceded the appearance of embolism in the leaves and the stem by several days. Basal leaves were more vulnerable to xylem embolism than apical leaves and, once embolized, were shed, thereby preventing further water loss and protecting the hydraulic integrity of younger leaves and the stem. As a result, embolism in the stem was minimal even when drought led to complete leaf shedding. These findings suggest that grapevine avoids xylem embolism rather than tolerates it.

Plants are strongly influenced by their thermal environments, and this influence manifests itself in a variety of ways, such as altered ranges, growth, morphology, or physiology. However, plants also modify their local thermal environments through feedbacks related to properties and processes such as albedo and evapotranspiration. Here, we used leaf-and plot-level thermography on the north slope of the Brooks Range, Alaska, to explore interspecific differences in thermal properties among arctic tundra plants, and to determine if species differentially contribute to plot temperature. At the leaf-level, we found significant differences (p < 0.05) for in situ temperatures among the 13 study species. At the plot level, we found that the fractional cover of vascular plant species, lichen, litter, and moss had a significant effect on plot temperature (p < 0.05, R-2= 0.61). A second model incorporating thermal leaf properties-in addition to the fraction of vascular plant and other dominant ground covers-also predicted plot temperature, but with lower explanatory power (p < 0.05, R-2= 0.32). These results potentially have important implications for our understanding of how individual plant species influence canopy-level thermal properties and how temperature-dependent properties and processes may be impacted by climate change-induced shifts in species composition.